Ice Sheet Formation
—Jeni Casinelli, Education and Program Coordinator
How does ocean ice form? What does it look like? Does ice formation occur the same way all over the world? How does the salt affect ice formation? These are just a few of the questions that come to mind when thinking about where ice sheets come from.
Once ocean temperature drops below 29 degrees Fahrenheit, small crystals called frazils begin to slowly form below the ocean's surface. Each frazil is about 3-4 mm in diameter. As frazils form they float to the surface, meeting up with other frazils and beginning to form ice sheets. At this point the formation of ice sheets takes two very different pa
ths, depending on the state of the ocean waters.
In calm waters, the frazils join together forming thin sheets of ice that look similar to oil slicks, called grease ice. At first they appear almost black in color, but turn whiter as the sheets begin to overlap, and form a thick ice sheet. In rough waters, the frazils join together into circular disks called pancake ice. As ice formation continues, the pancake ice collides and overlaps into thick sheets.
Do the two ice sheets look the same? Not quite. Calm water ice sheets will result in smooth-bottomed sheets while rough water formation will result in rough-bottomed sheets from the constant collision of the pancake ice.
While ice formation occurs, what is happening to the salt within the water? As salt cannot freeze, it is expelled during ice formation, leaving behind ice crystals that are purely fresh water. Almost three-fourths of all Earth's fresh water is stored within ice sheets and glaciers. As the salt is expelled, it is released to the underlying ocean causing the upper ocean layer to increase in density. As denser items sink, this increase in density leads to denser bottom water formation. When ice melts, fresh water is released, creating a more consistent density throughout.
Today, the majority of Earth's ice is located within the ice sheets covering Antarctica and Greenland. The Antarctic ice sheet alone covers more than 5.4 million square miles of Earth's surface and together the two ice sheets hold 99 % of earth's freshwater ice. The melting of land based ice sheets could cause potential problems for low-lying areas around the globe including coastal flooding, erosion along the shoreline, and freshwater contamination.
